In order to assess the changes in the cellular immune system, 27 healthy male subjects were selected who participate in leisure-time sports and in whom measurements of the lymphocyte subsets in venous blood were carried out before and after a standardized endurance exercise test (EI, 60 minutes in length, intensity: 63% of maximum performance). The percentages of the following parameters were measured: total T-lymphocytes, B-lymphocytes and O-lymphocytes as well as T4-lymphocytes (helper T-lymphocytes, CD4+) and T8-lymphocytes (cytotoxic/suppressor T-lymphocytes, CD8+). In a subgroup consisting of 8 subjects, the parameters indicated above were measured again 24 hours following E. In order to check the reproducibility of the findings, the endurance exercise test was repeated 21/2 weeks later (EII).